Why does a flashing fail when the covering is still sound?
Because the flashing is a joint, and joints move, age and crack in the sun and the frost. The covering can be fine and the joint still leak.
Flashing is made of metal, rubber or membrane, and each of these expands and contracts with the temperature. The sealant that holds it moves with the material and cracks where the movement is greatest. Over years the joint opens, and water follows the opening under the covering. The shingles beside the flashing can look perfect while the joint behind them lets the water through.
Storm wind adds to the problem. Wind lifts the lap of the flashing, and a lap that was marginal in calm weather can open in a gale. That is why the joints are checked after a storm as well as in the normal inspection.
- Movement from heat and cold cracks the sealant first
- Wind lifts a marginal lap in a storm
- A sound covering can hide a failed joint
What is the right sealant and when is it used?
The sealant the flashing detail names, applied to a clean dry surface and not used as a substitute for a correctly lapped joint.
Sealant is a secondary defence. A flashing that is lapped correctly sheds water without it, and the sealant only fills the small gaps that remain. A joint that is held together with sealant alone is a joint that will fail when the sealant does.
The roofer names the product in the scope. The product is chosen for the material it joins, and it is applied to a clean, dry surface. A sealant on a wet or dusty surface does not bond, and the joint leaks within a season.
- Lap the flashing so water sheds without sealant
- Use the product the detail names for that material
- Apply to a clean, dry surface only
When should flashing be replaced rather than repaired?
When the metal is corroded through, when the lap has been set wrong from the start, or when the flashing has been patched several times.
Repairing flashing is quicker than replacing it, and most lifted or cracked joints can be reset and sealed. A flashing that is rusted through, that has been patched more than once or that was set without the right lap needs replacing. Patching a failing flashing repeatedly costs more over time and leaves a joint that no one can trust.
A roofer who replaces the flashing will say so in the scope, name the material and explain the lap. Ask for that detail to be written down so the next roofer can see what was done.
- Metal rusted through or split along the fold
- Flashing patched more than once at the same joint
- A lap that was set the wrong way from the start
How should flashing be inspected after a storm?
Look at each joint from the safe side, check the flashing for bent or lifted edges, and test any joint where water is suspected with a hose from above.
After a storm the flashing at walls, chimneys and vents is the first place to look. From a safe position, check whether any piece has been bent up, lifted or pulled out of its reglet. A bent edge is a sign the joint has taken wind or hail, and it needs resetting even if no water has come through yet.
Where water is suspected, a hose test from above, one joint at a time, is the most reliable check. Someone should watch the wall or ceiling below, so the joint that lets the water through is found and not guessed. The result is noted with the date, so the next storm can be compared against it.
- Look for bent or lifted flashing from the safe side
- Reset any bent edge even before a leak shows
- Test a suspect joint with water, one at a time
